“Photonic crystal” is an artificial novel crystal that has been proposed in recent years and has been attracting attention ever since (E. Yablonovitch, Phys. Rev. Lett., 58 (1987) 2059–2062). It can be obtained by periodically arranging substances having different refractive indexes at intervals substantially equal to the optical wavelength. Research and development efforts are being paid to produce optical elements out of such a crystal substance because it has peculiar optical characteristics including those of showing a photonic band gap and having apparently abnormal refractive indexes that are attributable to its so-called photonic band structure resembling the band structure of a semiconductor. It can also be engineered artificially in terms of structure and scale.
Active-type optical elements are among such optical elements that are worthy of paying attention. An active-type optical element is an element at least one of whose optical characteristics can be externally and actively controlled not only in the design stage but also while it is being used. Expected applications of active-type optical elements include variable filters, optical switches and many other optical devices.
Japanese Patent Application Laid-Open No. H10-253829 proposes a method of arranging an actuator at the periphery of a fiber diffraction grating and expanding/contracting it to apply tensile force to the fiber so as to control the distribution of refractive indexes in the fiber.
Japanese Patent Application Laid-Open No. 2001-091911 proposes a method of introducing a substance such as a piezoelectric element, with which the refractive index and the transmittivity can be controlled externally, into photonic crystal and disturbing the periodicity of crystal by utilizing expansion/contraction or changes in the characteristics of the substance.
WO02/27384 proposes a method of controlling the lattice spacing of photonic crystal by applying pressure externally to it.
However, these known techniques are accompanied by drawbacks as listed below.
Firstly, the method of expanding/contracting an optical fiber requires a member for transmitting force to the fiber in addition to a member for generating expanding/contracting force that may be a piezoelectric element because the one-dimensional periodic structure arranged in the direction of incident light needs to be changed. Then, there arises a problem that the precision of controlling the lattice spacing depends on the material, the arrangement and the connection of the force transmitting member. Additionally, while apparently abnormal refractive indexes as described above appear in a periodic structure that is two or higher dimensions, the overall arrangement becomes complex when force needs to be applied in two or more than two directions by way of a force transmitting member.
The method of introducing a means for disturbing structure of photonic crystal into the latter requires a large number of processing steps and the scope of materials that can feasibly be used is limited.
Finally, the method of externally applying pressure to a photonic crystal to change the crystal structure thereof is accompanied by a problem that, as pressure is applied to a photonic crystal showing a periodic distribution of refractive indexes as shown in FIG. 5 (vertical parallel lines indicate the period in FIG. 5) to deform the crystal, quantities of deformation are accumulated from the center toward the periphery to remarkably distort the entire crystal as shown in FIG. 6. Additionally, the characteristics of the crystal can appear unevenly at different sites in the crystal. Furthermore, changes in the distribution of refractive indexes within a period, those in the period and those in the phase coexist to make it difficult to control the crystal structure.
In view of the above identified circumstances, it is therefore the object of the present invention to provide a method of controlling an optical characteristic of an optical element that can improve the dimensional precision of the entire optical element, unify the optical characteristics and enhance the degree of freedom of choosing materials and also an optical device that can be used with the method.